Donald Trump's decision to become the first president ever to attend the annual March for Life in Washington on Friday is a very big deal for both the Republican Party and the pro-life movement — though not in the way that either of them fully realizes.

Trump's embrace of the anti-abortion movement has been driven from the beginning by political expediency. With various factions of the Republican Party establishment opposing him in the 2016 primaries, the Trump campaign made the decision to champion the religious right, including its pro-life activist base, which then reciprocated by turning out to vote. As president, Trump has rewarded this support with White House access and respect for the movement's leadership, as well as record numbers of staunchly conservative judicial appointments that just might lead to decisions upholding restrictions on abortion that render the key abortion rights decisions, Roe v. Wade (1973) and Casey v. Planned Parenthood (1992), effectively null and void.

Still, at the symbolic level nothing has come close to matching Trump's announcement on Wednesday that this year he would be attending (and presumably speaking at) the March for Life, the demonstration in the nation's capital that is held annually on or close to the anniversary of the Roe decision and regularly draws tens of thousands of pro-lifers.

The significance for the GOP of Trump's scheduled appearance at the rally is a function of what it demonstrates retrospectively about the place of the pro-life movement in the party. Ronald Reagan and George W. Bush may have been personally committed to the anti-abortion cause, and Trump may be entirely driven by concern over his prospects for re-election and his need to keep Republican office-holders firmly in his camp through the Senate impeachment trial. Yet the former presidents kept the movement at arm's length and the latter has unapologetically embraced it. Donald Trump is now poised to be the greatest presidential champion of the pro-life cause in American history.

That's a big change for the GOP, which has long tried to have it both ways — doing just enough to win loyal support from grassroots pro-life activists but not so much that it antagonizes pro-choice voters who have been willing to cast ballots for the party despite its nominal support for the anti-abortion cause. Such balancing will now be much more difficult if not impossible with the head of the party so forthrightly embracing the pro-life position.

But Trump's unmodulated support of the pro-life cause is a much bigger and more portentous change for the movement itself.

Nationwide opposition to abortion began with outspoken Catholic bishops immediately after Roe was handed down and was at first joined by greater numbers of Democrats than Republicans. These activists originally saw themselves as leading a civil rights crusade in favor of defending the most vulnerable members of society (the unborn) against lethal violence. This construal of the cause has animated and sustained it for nearly 50 years, long past the realignment that saw it become a foundational issue for a religious right dominated by evangelical Protestants.

All along, critics of the anti-abortion movement have dismissed its moral appeals and deployment of the language of civil rights, calling it window dressing on a cause that is really motivated by hostility to feminism — and above all, by the drive to limit women's autonomy, stymie their career ambitions, and reaffirm the primacy of their social role as mothers.

For decades the pro-life movement has fought this characterization. In doing so, it has drawn on the rhetoric of such anti-abortion crusaders as Richard John Neuhaus, who adapted language from his days marching for civil rights with Martin Luther King, and the rationalist and universalistic moral arguments of legal philosopher Robert P. George, who has appealed to the tradition of natural law theorizing in forging his arguments against abortion.

The moral message of these men is simple and powerful: The fetus is a member of the human species from the time of conception; all members of the human species, no matter how small, weak, or dependent, possess innate dignity and hence a right to life; all abortion is therefore the taking of an innocent human life and so must be outlawed along with other forms of murder. This was a line of argument about the rights and legal status of the fetus, not one about the proper place of women in American society.

But a pro-life movement whose greatest political champion is Donald Trump has no hope for maintaining such a high-minded construal of its motives and priorities. When Trump speaks at the March for Life, the country will see the pro-life cause forthrightly advocated by a serial adulterer who's repeatedly been accused of rape, who cheated on his wife with a porn star shortly after the birth of their son, and who has been caught on tape bragging about his facility at sexual assault. Trump has spent a lifetime using women for his own gratification and then tossing them away like garbage — and he will now be the most powerful and prominent promoter of a movement that would prevent the victims of such treatment from asserting some modicum of control over its consequences on their bodies and future lives.

That's a pro-life movement that actively aims not to protect the vulnerable from lethal violence but to severely constrain and restrict women's freedom.

The practical consequences for the movement are likely to be bad. While some recent polling indicates a modest rise in the popularity of the pro-life label — showing perhaps that continuing advances in ultrasound technology are increasing moral unease about opting for abortion — other polls show a more dramatic spike in support for upholding Roe and the constitutional right to choose. This is a powerful sign that, as we've also seen with shifts in public opinion in favor of immigration over the past few years of nativist cruelty and xenophobia from the administration, Trump tends to turn people against the very causes he champions.

By locking arms with a polarizing, deeply unpopular, and morally repulsive president, the pro-life movement is likely to end up losing at the very moment it feels closest to winning.